This invention relates generally to process control systems, and more particularly to a system in which a time-sharing technique is used to exchange process information between controllers or computers and process input and output units over a common data line.
Basic to the conventional process control system is an electronic controller, such as the PID type (Proportional, Integral, Derivative). In a process loop, this controller acts to compare a process input variable, such as flow rate or temperature as sensed by a transducer, with a set point value to derive a deviation signal that depends on the difference between these values. The controller operates on the deviation signal by a control action such as proportional or integral action. In adjusting this action, the objective is to produce an output signal that will govern a final control element, such as a valve, to minimize both the duration and magnitude of the deviation signal resulting from a process load change or a change in the set point.
In recent years, process control systems have been developed which include a multiplicity of process control loops, each having a process input unit, a process output unit and a controller. To obviate the need for complex wiring arrangements, the exchange of information between the input and output units and the controllers is effected over a common data line by a time-sharing technique.
In one known process control system of this type, the arrangement is such that each of the process control loops is arranged to operate in accordance with a signal exchange sequence which is in cyclic form for multiplexing, so that the process input fed into the process input unit is caused to coincide with a set point fixed by an operator at an interface station, the final control element in this loop being governed by the process control output supplied through a process output unit from a computing unit or controller.
With a conventional multiplex process control system of the type heretofore known, the signal exchange sequence is fixed and one cannot alter the control mode, Thus the cyclic control routine is inflexible and cannot be modified at the option of the operator.